The yeast genes RNA2 through RNA11 are necessary for the processing of nuclear precursors in messenger RNA. Temperature sensitive recessive mutations in these genes result in the accumulation at the nonpermissive temperature of unspliced intron-containing precursors of mRNA. Evidence is accumulating that these RNA genes are directly involved in splicing. The RNA are in the yeast nucleus and temperature sensitive mutations in these RNA genes inactivate splicing extracts in vitro. A combined genetic, molecular biological, and biochemical approach will be taken to determine the function of the RNA2, RNA3, RNA4, and SRN2 genes in yeast mRNA splicing. Using the cloned RNA genes and antisera directed against the RNA gene products the following questions will be addressed: (1) Are the RNA gene products in the splicing complex - the spliceosome, or in snRNPs or HnRNPs? (2) What other molecules interact with the RNA gene products, and are they involved in mRNA splicing? (3) How do the RNA3 and RNA4, and SRN2 and RNA2 genes interact with each other? (4) Which steps in the assembly and function of the splicing complex require the function of RNA genes?